'ChangeWorker' method needs to be delayed

Since we use the worker key to sign blocks, any change in this address represents an entirely different probability set for block production. I believe snow white mentions this attack.

We should delay a change in the worker key by at least K (the randomness lookback). I think double that is probably a good idea, in case people try to fork off and mine their own chain where they win more.

As for the specific mechanism, i'd like to avoid having to have two method calls to change the worker key (`StartKeyChange` and `FinalizeKeyChange`). So maybe we can tie it to PoSt submission somehow?

I'll sleep on this and come up with something tomorrow.